Match Me If You Can
Mar 1, 2003 12:00 PM, By Pete Putman, CTS
1:1 pixel mapping is a hit-or-miss job with today’s projectors and monitors.
 Sanyo’s PLV-Z1 front projector’s unusual 964x544 pixel count is one of many resolutions difficult to match to video content. |
A long time ago, in a production studio far, far away, there were video monitors. And all of these monitors used cathode-ray tubes (CRTs). In this distant time and place, we didn't worry about how much resolution these monitors could provide; we just turned them on and calibrated them as best we could to display 525-line (or 625-line) interlaced, standard-definition video. Back then, if we ever discussed the picture resolution of a CRT monitor, we spoke of “electron gun beam spot size” or “shadow masks.” Things have changed.
The explosive growth of fixed-pixel imaging technology over the past decade has introduced a new phrase to the industry lexicon. Now, when we refer to projectors and monitors, our attention is focused more on the projector's or monitor's “native resolution” — a measurement derived from a specific horizontal and vertical pixel array.
Sounds simple enough, doesn't it? As usual, the devil is in the details. The process of matching up video-standard pixel resolutions and computer-standard pixel resolutions is often akin to fitting a square peg into a round hole. Indeed; it is a rare occurrence when the path from image source to screen is straight and true with no format conversion whatsoever.
Every solid-state imaging system in use today, with the exception of Sony's Grating Light Valve (GLV) technology, has some sort of pixel matrix and functions either as a light shutter (digital light processing, transmissive or reflective liquid crystal) or as an emissive light source (plasma and organic light emitting diodes).
Those pixels are almost always square in shape (there are some exceptions), and the pixel array usually conforms to a PC display standard or variations of it. The LCD imagers used in front projectors and rear-projection monitors typically offer XGA (1024×768 pixels) or SXGA (1280×1024 pixels) resolution, while front projectors add SVGA (800×600) pixels to that list.
There are variants on these formats. The SVGA digital micromirror devices made by Texas Instruments have a native resolution of 848×600 pixels. JVC's SXGA Direct Drive Image Light Amplifiers (a fancy name for liquid crystal on silicon) sport 1365×1024 pixels. And there are now oddball LCD imagers coming to market with non-standard pixel counts like 964×544, found in Sanyo's PLV-Z1 front projector.
Want more examples? In the DMD world, we'll find 848×480 and 1280×720 pixel devices, while there are now 858×488, 960×520, 1280×768, and 1366×768 polysilicon LCDs. Toshiba recently introduced a rear-projection TV that is supposed to use 1920×1080 pixel LCoS devices, and JVC now offers a 2048×1536 QXGA D-ILA panel in addition to its newest 4×3 offering, which measures 1440×1050 pixels and is labeled “SXGA Plus.”
Confused yet? It gets worse. Plasma and LCD monitor and TV manufacturers have adopted a crazy quilt of pixel arrays, from Wide VGA (852×480 or 853×480) and Wide XGA (1280×768, 1365×768, and 1366×768) to Fujitsu's and Hitachi's ALiS designs (852×1024 and 1024×1024) and the new 1024×768 non-square format adopted by NEC, Panasonic, and Pioneer. Note that all of these are 16×9 panels, even the last two which use non-square pixel shapes.
Stop making sense
And you thought the 18 different ATSC standards were confusing. This “resolution confusion” problem is further exacerbated by a trend among manufacturers of CRT-equipped, HDTV-ready direct-view and rear-projection monitors to use one and only one horizontal scan rate — 33.75kHz, the standard for 1080i/30. The reason? It keeps the cost and complexity of components down.
This means that while 1080i HDTV is shown at its native scan rate on these TVs and monitors, all other signals are format-converted to 540p/60 (which is, conveniently, one-half of 1080i). The oscillator frequency never changes, but progressive-scan signals go “up” from 480p or “down” from 720p before making it to the screen. To accomplish this, picture information is either sampled and pixels are increased or decimated as required.
The issue of what “native” rate to use came up recently when I received an email asking whether the writer should set the output of his new Zenith HDV420 terrestrial DTV set-top tuner to 1080i or 720p before connecting into a 42in. plasma monitor. In essence, should he format-convert before connecting the signal, or let the plasma panel do the funky math? (Keep in mind that his panel, a Panasonic WVGA design, had only 852×480 pixel native resolution, so pixel decimation was inevitable somewhere along the line.)
So, we have video and PC signals arriving in one format that may or may not be converted to a second format for display. And, in some cases, a de facto third format conversion comes into play! The picture tubes and projection tubes used in most HDTV-ready monitors and TVs do not have sufficient resolving power to show all of the detail in 1080i images. At best, they are 810i or 960i devices.
There's also the issue of converting interlaced scans to progressive scan. 1080i sources are so affected, as are all NTSC or PAL video feeds, meaning motion artifacts and possible 3:2 pulldown to correct. (The ALiS plasma panels use an alternating line presentation of progressive-scan picture information to reduce power consumption.)
To WYSIWYG or not?
Sometimes we get lucky with matching source to display. One example that jumps to mind is progressive-scan DVD playback at 704×480 pixels, a very close match to the 848×480 or 852×480 WVGA standards for some projectors and monitors. While not a true 1:1 pixel map, it's darn close and the image quality can be exceptional.
Another good example is 720p HD content viewed on one of the new front projectors or RPTVs using Texas Instruments' Mustang/HD2 DMDs. These have the exact same resolution (1280×720 pixels) and — assuming you have a set-top box, D-VHS, hard drive recorder, or other playback system with true 720p output — the picture quality can be outstanding.
Until recently, there haven't been any practical implementations of devices capable of 1920×1080 pixel resolution. But Toshiba's new LCoS TV uses panels with a 1:1 map, as does LG Philips' yet-unbranded 52in. LCD monitor and Samsung's new 54in. LCD display. However, unless you have one of these products, you'll have to settle for a pixel format conversion to match your monitor or TV.
For 1280×720 devices, displaying 1080i content means a pixel decimation and picture re-map to 66% of the total available pixels. The good news is that it's almost a linear conversion in both horizontal and vertical. Re-mapping 1080i to 1365×768 WXGA for plasma is also a linear down-conversion of 29% (and that's close enough for 1366×768, too).
It takes a bit of nonlinear math to fit 1280×768, a common standard for LCD TVs and Pioneer plasma monitors. The horizontal pixel decimation is about 33%, but the vertical pixel resizing is only 29%. For 1024×1024 ALiS panels, it gets even stranger! The vertical image map is within 5.2% of full resolution, but the horizontal pixels must be sampled and compressed by 47%. This is why 1080i HDTV often looks crisper on lower-resolution panels that employ a linear reduction and pixel decimation.
And of course, those 1024×768 non-square PDPs require an equally bizarre re-mapping process. Consider that a 4×3 1024×768 source image from a PC must be resized to 767×768 non-square pixels for native display, while a 1920×1080 HD signal is decimated by 47% horizontally and 29% vertically! It's a daunting task, but can be pulled off with premium image scaling.
Who designed this mess?
It's easy to assume that the designers of some of these projectors and monitors were under the influence of hallucinogens. But there's a better explanation, and that's the physical limitations of the fabrication process.
There is tremendous market pressure to bring “HD” resolution to all consumer and professional displays. 480p just doesn't cut it anymore, and 800×600 (or 600p) imaging only survives in front projectors and some RPTVs because of attractive low prices. In a world where the majority of computers have settled at 1024×768 pixel resolution and the lowest HDTV picture display standard is 1280×720p, products with lower resolution just aren't as attractive.
The unique 1024×1024 pixel matrix used by the Fujitsu-Hitachi plasma (FHP) factory on Japan's Kyushu Island came about because its plasma fabrication line couldn't handle glass sizes larger than 42in. So, to increase pixel density and resolution, the engineers came up with a pixel shaped like a brick standing on end. The smallest they could make this pixel was .90mm × .51mm (which, oddly enough, works out to an aspect ratio of 1.7647:1, or close to 1.78:1).
At that size, FHP could cram 1,024 of them in both axes and call the finished 42in. panel an “HDTV monitor”. Of course, this set off a controversy within the Consumer Electronics Association (CEA) as to what exactly an “HD” monitor was, particularly since the CEA's own definitions call for 1:1 mapping of at least one HDTV standard (usually 1280×720).
FHP also designed a 32in. panel that had a native pixel count of 852×1024 pixels in a 16×9 aspect ratio. (These pixels measure .84mm × .39mm). This panel is also called an HDTV monitor, even though the highest resolution it can map close to 1:1 is 1024×768 in a non-square format. The 1024×768 plasma monitors from Panasonic, NEC, and Pioneer contain non-square pixels with an unusual size all their own (NEC's pixels measure .897mm × .657mm).
Can't anybody match my display?
Manufacturers of 50in. and 60in. plasma sizes haven't been as constrained. Their pixels can be larger — often approaching or even exceeding 1mm in pitch — so it's much easier to come up with a proportionate, true HD pixel count. Widescreen LCD monitors and LCD TVs can use smaller pixels with the result that all LCD TVs above 22in. are now at 1280×768 pixel resolution. The notable exception is Samsung's LTM245W 24in. LCD TV, which has a native resolution of 1920×1200 pixels.
But there are still very few projectors and monitors that match HDTV display formats exactly. The trend has always been to conform to PC display standards and widescreen variations, and unfortunately none of the ATSC HDTV formats exactly corresponds to PC standards. They come close, but no cigar.
Is this a big problem? From the professional and industrial perspective, the actual pixel count matters only if you are performing image manipulation and need to see as much detail as possible. For colorists, digital transfers, and animators, the goal should be to match the resolution of the display as closely as possible to the source resolution. (Issues with white balance and grayscale on projectors and monitors are best left to a future discussion.)
If this isn't possible, a display that performs a linear pixel conversion in both axes will yield the sharpest and cleanest images. (For this reason, 720p and 1080i HD content both look cleaner and crisper on 852×480 plasma than they do on 1024×1024ALiS panels!) If you are merely editing, mixing, dubbing, or otherwise synching up picture and audio elements, the actual resolution of your projector or monitor isn't as important.
Corporate and educational users aren't as fussy, either. Because most current-model projectors and monitors can map XGA 1:1 and do a good approximation of SXGA, that satisfies most of their needs. For public display and signs, the actual resolution of plasma and LCD monitors is almost irrelevant because text, photos, and graphics are usually VGA or SVGA, either mapped 1:1 or scaled up to fit available pixels.
The most finicky customers are home theater enthusiasts, many of which go to great extremes to precisely match all of their video sources to plasma TVs and front projectors. Some of these displays have DVI inputs, but there are few DVI sources available to drive them at full resolution. Some exceptions are video scalers made by Key Digital, Focus Enhancements, Faroudja, and others.
For your purposes, you are best off mapping your source images as closely as possible to the full pixel count of the projector or monitor you've purchased. All of these products have internal PC and video scalers that provide mixed results — down-rezzing usually looks a lot better than up-rezzing — but there are plenty of aftermarket scaler and scan converter “solutions” to be had that can clean up these pixel re-mapping jobs.
Help is on the way (sort of)
The solution to this tangled web is to move toward all-digital interfaces like DVI. Granted, there will still be a fair amount of image scaling going on to make the tricky handshake work between TV and PC standards.
But it's possible that newer, smarter chipsets with a large number of display standards in memory will be able to perform the necessary scaling while minimizing artifacts.
If you are considering purchasing a video scaling product to drive your new DVI-equipped projector or plasma/LCD monitor, I strongly suggest you choose from models equipped with DVI-D outputs as opposed to depending on analog component signal interfaces. There is a noticeable difference in image quality by sticking with an all-digital path, particularly if the scaler also supports SDI or HD-SDI inputs.
For everyday viewing of video and HDTV, it's best to match the optimum resolution or scan rate of your TV, monitor, or projector. I use a Toshiba 34HF81 34in. flat-screen CRT to watch HDTV in my family room, and the only HD rate it supports is 1080i. 720p is ignored altogether, and using 480p requires my changing the TV's aspect ratio setting. So, my Samsung SIRT-165 DTV set-top receiver is set to convert all received DTV programs to 1080i output for convenience, highest resolution, and practicality.
On the other hand, my home theater uses a Sony VPH-D50HTU three-tube CRT projector, which is a true multiscan device. In this case, I leave my set-top tuner (Panasonic TU-DST51A) set to “native” format output, which lets me watch 480p, 720p, and 1080i programs the way they were encoded and sent out by the networks. Even 480i DTV programs pass through and appear unadulterated, scan lines and all!
For the future, the holy grail is to employ imaging matrices with full HD resolution of 1920×1080 pixels, or something close to it. But that's still a long way off, despite what Toshiba and JVC have achieved. Until then, manufacturers of projectors and monitors will continue to use PC-based standards, while video source material is created in 480i, 480p, 576i/p, 720p, and 1080i/p digital formats.
Pete Putman is president of Roam Con-sulting Inc. of Doylestown, Pa. He can be reached at vspete@projectorexpert.com.
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